1. Field of the Invention
The present invention relates to a light-emitting module, especially, a module for a wavelength division multiplexing (WDM) system.
2. Related Prior Art
In the WDM communication system, the minimum wavelength interval between respective signal channel is set to be 0.8 nm or less, this means that the absolute accuracy of the wavelength is controlled within ±0.1 nm. To realize such absolute accuracy, temperatures of respective optical sources is controlled individually by a feedback loop. Namely, the temperature of the semiconductor laser of respective channel is monitored, is compared with a preset value, and adjusted by changing the current providing to the Peltier element because the wavelength of light emitted from the semiconductor laser depends on the temperature.
Conventional light-emitting module is shown in FIG. 18. The module comprises a semiconductor laser, a thermoelectric cooler such as a Peltier element, an optical fiber and a housing, the laser and the Peltier element encases therein. The laser is mounted on the Peltier element so as to control a temperature of the laser. The Peltier element is driven from the outside of the housing. To apply such conventional module to the WDM system, additional circuits must be provided for respective channel outside the module housing, thus results in a complicated and a large sized system.
To realize a compact sized module and a simple optical source system, it is necessary to encase the Peltier driver circuit in the housing. However, as shown in FIG. 18, the conventional housing has no rest room to put the circuit therein.